A liquid polysulfide polymer has thiol groups at the terminal end and is easily oxidized by an oxidizing agent such as lead dioxide and manganese dioxide and cured. A cured rubbery material obtained by curing a polysulfide polymer contains sulfur and does not contain a double bond in the main chain of the molecule, thus is excellent in oil resistance, weather resistance, water tightness and gas tightness, and is also good in adhesion. Thus, it is widely used as a sealant, adhesive and paint.
A liquid polysulfide polymer has high compatibility with an epoxy resin, and can be mixed with an epoxy resin at various rates, and a curable composition can be obtained by adding an amine to the liquid polysulfide polymer. Polysulfide polymer addition to an epoxy resin has an effect of providing flexibility to a hard and brittle epoxy resin cured material to improve adhesion. It is known that, when a liquid polysulfide polymer is added to an epoxy resin composition as a primer to join old and new concrete, adhesion to a wet surface is improved, and cohesion after being cured is good. When a polysulfide polymer is added to an epoxy resin, adhesion to a hardly adhesive rubber is improved. Furthermore, when a mixture of an epoxy resin and a polysulfide polymer is added to a sealant, resistance of the sealant is improved for long periods.
The most common method of producing a liquid polysulfide polymer is a method obtaining a liquid polymer via solid polysulfide described in U.S. Pat. No. 2,466,963.
In the method of producing a liquid polysulfide polymer via solid polysulfide, molecules are cleaved to form terminal groups and, at the same time, solid is liquidified. When the thiol group content is increased, the molecular weight of the liquidified polymer is decreased, but control of the molecular weight becomes difficult, and the content of thiol groups is consequently restricted. Furthermore, when attempting to increase the thiol content at the terminal end, a large amount of a liquidifying agent is necessary. Thus, there are problems that the production cost is high, and a large amount of high-load wastewater is generated from the process and adversely affects on the environment. In addition, qualities such as color and smell also deteriorate. Therefore, the thiol group content that can be produced as a business has been limited to currently commercially available “Thiokol LP-3” (thiol group content, 5.9 to 7.7% by weight) having a viscosity range of 990 to 1340 mPa·s (25° C.).
A commercial product of a liquid polysulfide polymer produced by an existing production method has a long main chain relative to the thiol group. Thus, the reaction with an epoxy group is mild and, sometimes, hardening is too slow and, particularly, an increase in hardness is insufficient, depending on the intended use. In addition, in a commercial product of a liquid polysulfide polymer produced by an existing production method, sometimes, the viscosity is too high. Thus, an organic solvent that poses environmental concerns is necessary, depending on the intended use.
As a compound that can rapidly react a thiol group with an epoxy group, terminal-thiol group-containing compounds that do not contain a polysulfide structure in the main chain have been reported. Among them, as a curing agent of an epoxy resin having both economic efficiency and safety, a compound having a polyether structure in the main chain and having three or more thiol groups in one molecule is widely commercially available. Examples include “Polythiol QE-340M” manufactured by Toray Fine Chemicals Co., Ltd., “Capcure 3-800” manufactured by BASF Japan Ltd., and “GPM-800” manufactured by Gabriel Performance Products. These terminal-thiol group-containing polyether compounds have an average thiol group content of 12% by weight and viscosity of 7000 to 18000 mPa·s (25° C.). These commercial products of terminal-thiol group-containing polyether compounds are used for various usages, and characterized in having high thiol group content and having high hardenability. However, since these compounds do not have a polysulfide structure in the main chain, there are problems of poor performance in flexibility, water resistance, chemical resistance, weather resistance, and adhesion compared to when a terminal-thiol group-containing liquid polysulfide polymer is used.